Replace PHPStan with drupal-check for deprecation testing

[TravisCarden]

This replaces PHPStan installed directly in the fixture with drupal-check installed in ORCA itself, for two benefits: 1) it improves the resiliency of deprecation testing, making it less susceptible to total failure from errors in tested code, and 2) it improves the production-likeness of the fixture by removing packages that aren't strictly necessary and can't be assumed to be present in a customer build.

If you were depending on PHPStan being in the fixture, you can add it back yourself in a custom script:

#!/usr/bin/env bash

# Set the current working directory to that of the script so relative paths
# are resolved predictably.
cd "$(dirname "$0")" || exit 1

# Reuse ORCA's own includes for its $PATH additions and environment variables.
# This example assumes that your script is at a directory depth of two (2), e.g.,
# my_module/bin/travis/before_script.sh
source ../../../orca/bin/travis/_includes.sh

# Move to the fixture directory if present or exit early if not, as in the case
# of the static analysis job, for example, which creates no fixture.
cd "$ORCA_FIXTURE_DIR" | exit

composer require mglaman/phpstan-drupal-deprecations nette/di:^3.0